Experimental analysis of mixed mode crack propagation in brittle rocks: The effect of non-singular terms

The cracked chevron notched Brazilian disc (CCNBD) specimen, suggested by the International Society for Rock Mechanics for testing mode I fracture toughness of rocks, usually yields rather conservative toughness measurements, and the reasons have not been fully explored. In this study, the CCNBD method is compared with the cracked chevron notched semicircular bending (CCNSCB) method in the fracture process zone (FPZ) and its influence on the fracture toughness measurement. Theoretical analysis reveals that the FPZ is longer in the CCNBD specimen than in the CCNSCB specimen using a relatively large support span, the toughness measurement using the former is affected more seriously by the presence of FPZ, and thus the CCNBD method is usually, more or less, conservative compared with the CCNSCB method. These inferences are further validated by experimental results, which indicate that the CCNBD test indeed produces much lower fracture toughness values and even the results of 75‐mm radius CCNBD specimens are still lower than those of 25‐mm radius CCNSCB specimens. Consequently, due to smaller FPZ, the CCNSCB specimen with a relatively large span is more likely to produce comparably accurate or representative toughness value, and it may be more suitable than the CCNBD specimen for the engineering applications that require more representative or less conservative fracture toughness.

Section: Test Methods Loading Typementioning

confidence: 99%

An experimental and theoretical comparison of CCNBD and CCNSCB specimens for determining mode I fracture toughness of rocks

Wei

Dai

Liu

et al. 2017

“…In this regard, amongst the well‐known test specimens is the centrally cracked Brazilian disc (CCBD) under a concentrated diametrically compressive load. It provides a simple and effective technique to evaluate in‐plane shear and mixed mode I/II fracture of different materials …”

Section: Introductionmentioning

confidence: 99%

“…It provides a simple and effective technique to evaluate in-plane shear and mixed mode I/II fracture of different materials. [10][11][12][13][14][15][16][17][18] The CCBD specimen has a simple geometry and loading arrangement and, further, the capability of applying various mixed modes by changing the precrack inclination angle relative to the applied diametric load. For an inclined crack in compression, different regimes of contact between its surfaces can arise.…”

Section: Introductionmentioning

confidence: 99%

Modes I/II SIF of a diametrically compressed Brazilian disc having a central inclined crack with frictional surfaces

Hammouda

Fayed

2017

An in‐house finite element code was used to analyse a diametrically compressed isotropic homogeneous Brazilian disc having a central inclined crack with frictional surfaces for the evaluation of its corresponding modes I and II stress intensity factor (SIF). One‐half of the disc was successfully idealized. The coefficient of friction between the crack surfaces ranged from 0 to 1 in steps of 0.1. Relative crack length ratios of 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, and 0.7 were analysed with crack angles up to 90° in steps of 5°. The crack angle corresponding to the transition from a mixed mode I/II to a pure mode II decreases with increasing the relative crack length ratio. An increase in the relative crack length ratio leads to an increase in the range of crack angles corresponding to a partially closed crack. The crack angle at which fully closed sliding crack surfaces commences is 30° independent of the relative crack length ratio and the friction coefficient. With the same relative crack length ratio and increasing the friction coefficient, there is a corresponding decrease in the crack angle for the maximum mode II SIF. With a decrease in the relative crack length ratio or the friction coefficient, the crack angle beyond which the crack surfaces become in a fully sticking mode decreases resulting in zero mode II SIF. Generally, mode II SIF decreases with increasing the friction coefficient between the crack surfaces. Good agreement has been generally obtained with relevant results found in the literature.

“…where a β , b β and c β are the functions of crack inclination angles β in radian,when β ≤ 45°, a β ¼ À2:4808Âβ 3 þ 26:26Âβ 2 À 21:81Âβ þ 1:736 b β ¼ 2:805Âβ 3 À 27:98Âβ 2 þ 22:91Âβ À 2:499 c β ¼ À1:218Âβ 3 þ 8:062Âβ 2 À 6:314Âβ þ 0:5507 (9), (10) and (11). Fig.…”

Section: Calculations Of the Normalized T-stresses For Plates With Anmentioning

confidence: 99%

“…using a plate with a titled crack demonstrated that non‐singular terms in the series expansion of the stresses ahead of the crack had a significant influence on brittle fractures in elastic materials subjected to mixed‐mode loadings. Recently, the mixed‐mode fracture behaviour of Neyriz marble was studied theoretically and experimentally by using a centre‐cracked circular disk specimen and an edge‐cracked triangular sample by Akbardoost and Ayatollahi . A modified form of maximum tangential stress criterion, which takes into account the effects of non‐singular stress terms in addition to the singular terms, was used for justifying this difference and estimating the edge‐cracked triangular fracture resistance from the centre‐cracked circular disk one.…”

Section: Introductionmentioning

confidence: 99%

An investigation of T‐stresses in a plate with an inclined crack under biaxial loadings

Miao

Zhou

et al. 2016

For plates with an inclined crack of wide‐range aspect ratios under biaxial loadings, T‐stress values are calculated with three‐dimensional finite element method. The results show that the normalized T‐stress is crack length and orientation dependent. A linear equation for the relationship between normalized T‐stresses and biaxility factors is proposed to describe the normalized T‐stresses for different crack lengths and crack angles under different biaxial loadings, which is more convenient and involves wider biaxility ratios compared with the existing solutions. The plate thickness effect and the trend of normalized T‐stresses along the crack front thickness are also studied for mode I and I–II mixed‐mode cracks. Based on the analyses and comparisons, it is necessary to take the thickness effect into consideration when the crack length is long enough (a/W = 7/10). When the component of mode II is significant (β > 45°), and the biaxility ratios are negative, T‐stresses near the free surface are lower than those at other positions, which are the opposite of mode I crack and most of I–II mixed‐mode crack.